In a multistage planetary gear type speed reducer, a plurality of stages of planetary gear units and ring-like housing cases with internal gears divided correspondingly to the stages of the planetary gear units respectively are disposed coaxially and in parallel so that the housing cases with the internal gears can be combined and coupled with the stages of the planetary gear units respectively. One of such multistage planetary gear type speed reducers has been known as a cylindrical multistage type small-sized planetary gear speed reducer in which an output-shape-side flange portion, a plurality of stages of housing cases, and an input-side end cap portion are arranged continuously, and places where the stages of housing cases, the flange portion and the end cap portion are fitted to one another are fixed by joining/fixing units respectively to be thereby integrated (Patent Document 1).